The following patent applications, which are assigned to the assignee of the present invention and filed concurrently herewith, cover subject matter related to the subject matter of the present invention and are incorporated herein by reference:
09/791,651 Cradle For A Quick Barrel Change.
09/791,376 Force Isolating Cradle Assembly.
09/791,374 Injection Unit.
1. Field of the Invention
The present invention broadly relates to injection molding machines and, in particular to the injection unit of an injection molding machine. Injection molding machines include machines for injecting plastic material, or metal material, or metal material in a thixotropic state.
2. Summary of the Prior Art
Operation of an injection molding machine introduces a number of forces, pressures, and stresses on the injection unit. For example, axial carriage force is a force applied to engage the nozzle end of a barrel assembly against a sprue bushing of a mold. This provides a force sealing connection between the nozzle and sprue bushing preventing leakage of melted material during injection. Carriage force is applied and maintained prior to injecting the melt of material.
Injection force is a force directed along the length of a reciprocating screw located in a bore of a barrel assembly. Injection force results in injecting a melt of material into a mold. There is an axial reactive injection force acting along the length of the barrel assembly as a result of moving a screw forward during the injection stage of a molding process.
Injection pressure is a pressure required to overcome the resistance to the flow of the melt of material in the nozzle, runner system, and mold cavity. Injection pressure is exerted on the melt in front of the screw tip during the injection stage of a molding process. The accumulator end of a barrel assembly must withstand injection pressure.
Injection units for molding machines are very well known. For example, the book entitled xe2x80x9cInjection Molding machines A User""s Guide 3rd Editionxe2x80x9d by Johannaber was published in 1994 by Carl Hanser Verlag (ISBN 1-56990-169-4) and contains a detailed description of conventional injection units for plastic injection molding machines in Chapter 3 on pages 38, 39 42, 43, 44, 75, and 76. The reciprocating screw (RS) injection unit includes a barrel assembly which includes a nozzle, barrel head, barrel, axial bore, feed port, heater bands, and thermocouples. A reciprocating screw, which includes a non-return valve, is disposed in the axial bore of the barrel. The axial bore of the barrel includes a metering section and a feeding section. An electric or hydraulic drive operates the screw to feed and meter a melt of material and inject the metered material into a mold. The barrel assembly is fixed and supported, cantilevered, at one end of the barrel by a carriage. Hydraulic or electric actuators connect between the carriage and a frame member or fixed platen of the injection molding system. Operation of the actuators move the barrel assembly towards and away from the stationary platen and provides an axial carriage force through the entire length of the barrel during injection minimizing leakage between the nozzle tip and the sprue bushing. The axial reactive injection force is directed through the entire length of the barrel during injection.
The book entitled xe2x80x9cInjection Molding Operationsxe2x80x9d produced by Husky Injection Molding Systems Ltd., and printed in Canada, copyright 1980 also contains a description of conventional injection units for plastic injection molding machines on pages 41 through 44. Again, for the reciprocating screw injection unit, a barrel is supported at a distant end by a carriage, which houses the injection cylinder and a rotational drive. A hydraulic cylinder is connected between the carriage and a stationary platen. In operation of the hydraulic cylinder, the carriage force is applied along the entire length of the barrel. For a two stage injection unit, a barrel is supported at one end by a carriage. The carriage houses the drive. The nozzle of the barrel feeds into a shooting pot which includes an injection piston. The carriage supports another end of the shooting pot. A hydraulic cylinder is connected between the carriage and a stationary platen. In operation of the hydraulic cylinder, the carriage force is applied along the entire length of the shooting pot. The axial reactive injection force is directed through the entire length of the shooting pot during injection.
U.S. Pat. No. 5,040,589 issued on Aug. 20, 1991 to Bradley et al (assigned to The Dow Chemical Company). The patent describes an injection apparatus for injection molding a thixotropic semi-solid metal alloy. The patent contains a description of an apparatus for processing a metal feedstock into a thixotropic state as the metal is fed into a hopper, located at one end of the barrel, and transported into an accumulation zone located at another end of the barrel. The barrel is constructed of a single piece of material with thick walls. A number of heating zones are defined along the length of the barrel, including sections of the barrel having differing thickness. The feed throat area and zone 4 are relatively thick sections. Zone 3 is a slightly thinner section, and zone 2 is the thinnest section. The barrel is conventionally mounted in the injection unit. A feed throat end of the barrel is mounted in an upright support secured to the frame of an injection unit. A bottom surface of the barrel, intermediate the distant ends of the barrel, rests on a second support also secured to the frame. The carriage force is applied along the entire length of the barrel in operation of the apparatus. All sections of the disclosed barrel must be thick enough to withstand the combination of axial carriage force and axial reactive injection force directed through the entire length of the barrel during injection.
U.S. Pat. No. 5,983,978 issued on Nov. 16, 1999 to Vining et al (assigned to Thixomat Inc.). The patent describes a thixotropic metal injection molding apparatus. The barrel is formed in two sections to define a high pressure section and a low pressure section. The low pressure section is thinner than the high pressure section. A feed throat end of the barrel is mounted in an upright support of an injection unit. A bottom surface of the barrel, intermediate the distant ends of the barrel, rests on a second support also secured to the frame. The carriage force is applied along the entire length of the barrel in operation of the apparatus. All sections of the disclosed barrel must be thick enough to withstand the combination of axial carriage force and reactive injection force through the entire length of the barrel during injection.
There are a number of problems and deficiencies with the known prior art devices. Barrels are costly due to the amount of material required to provide a suitable thickness for withstanding the axial force along the entire length of the barrel. The axial force may be the carriage force, or the reactive injection force, or a combination of these two forces.
Special materials are required for barrels in use with thixotropic materials and these special materials are very expensive and are difficult to manufacture.
Thick barrels have a high thermal resistance which affects the efficiency and controllability of heating a material in the axial bore of a barrel.
Barrels, conventionally mounted in the injection unit, are typically difficult to install and remove. The process of installation and removal within a carriage is time consuming. Installation of the barrel in a carriage is further prone to alignment problems.
The primary objective of the present invention is to provide an improved barrel assembly for use in an injection molding machine.
A barrel assembly for use in an injection molding machine comprises a first barrel portion, a second barrel portion and a first barrel coupler disposed on said first barrel portion and isolating said second barrel portion from an axial force. The barrel assembly has an axial bore.
As an alternative, the first barrel coupler may include a linkage member. The first barrel coupler may include a second linkage member. The linkage member may include a thermal isolator. In an embodiment of the invention, the linkage member is a pair of standoffs. In another embodiment of the invention, the second linkage member is a ring.
As an alternative, the barrel assembly may include a second coupler disposed on the second portion of the barrel. The second coupler is adapted to cooperate with the second portion of the barrel and a second carriage coupler to permit axial movement of the barrel and prevent rotational movement of the barrel.
As an alternative, the barrel assembly may include an axial force linkage member disposed on the first coupler. The axial force linkage member distributes the axial force.
As an alternative, the barrel assembly may include a thermal isolator disposed on the first coupler. The thermal insulator reduces conductive heat transfer between the barrel assembly and a carriage.
As an alternative, the barrel assembly may include a linkage insulator disposed on the first coupler. The linkage insulator distributes the axial carriage force and reduces conductive heat transfer between the barrel assembly and the carriage. In an embodiment of the invention, the linkage insulator is an axial force linkage member and thermal isolator.
As an alternative, the barrel assembly may include a plurality of second couplers. In one embodiment of the invention, the second coupler is a recess formed in an outer surface of the second portion of the barrel. In another embodiment of the invention, the recess is a substantially flat pad. In another embodiment of the invention, the recess forms a spline. In another embodiment of the invention, the recess is an axially aligned slot.
As an alternative, the barrel assembly may include a plurality of axial force linkage members. In an embodiment of the invention, the axial force linkage member is of unitary construction formed on a surface of the first coupler. In another embodiment of the invention, the axial force linkage member is retained on the first coupler.
As an alternative, the barrel assembly may include a plurality of thermal isolators. In an embodiment of the invention, the thermal isolator is of unitary construction formed on the first coupler. In another embodiment of the invention, the thermal isolator is retained to the first coupler.
As an alternative, the barrel assembly may include a plurality of linkage insulators. In an embodiment of the invention, the linkage insulator is of unitary construction formed on a side of the first coupler. In another embodiment of the invention, the linkage insulator is retained to the first coupler.
As an alternative, the barrel assembly may include a barrel liner retained in the axial bore to isolate and protect the barrel from the melt of material.
The barrel assembly may be of unitary construction. Alternatively, the barrel assembly may be a plurality of barrel sections secured together. For the case wherein the barrel assembly is a plurality of barrel sections, each barrel section may further include a seal preventing leakage of a melt of material.
Further objects and advantages of the present invention will appear hereinbelow.